The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 2.3 Effect of force on velocity of a body.
Technical background 25
Fig. 2.4 Reactions to acceleration include the recoil of a gun and the thrust of the rotor that sustains a
helicopter.
2.4 Strength and rigidity
In aircraft, various forces must be resisted by the structure. The ability of a material
to resist forces is measured with respect to unit cross-sectional area, typically a square
metre. The force applied per unit of area is known as the stress. Figure 2.5 shows that as
the force applied to unit area increases, initially there will be a proportional elongation
according to Hooke’s Law. The elongation per unit length is known as the strain. The
constant of proportionality is known as the stiffness. In the linear region, the material
recovers its dimensions when the stress is removed. If the stress is further increased,
the strength of the material may be exceeded and an irreversible extension, known as
plasticity, may take place, leading finally to breakage. In brittle materials, the plastic
region may be negligibly small.
A structure that can resist a given static load may eventually suffer fatigue failure
if the load is repeatedly applied and removed. In practice a safety factor must be
applied to the maximum stress and the life of the component may be restricted to a
certain number of cycles. Fatigue primarily occurs in parts under tensile stress, probably
because this condition encourages the spreading of cracks. Highly stressed components